An example of a conventional optical semiconductor device is disclosed in a following patent document 1. FIGS. 7-9 illustrate the structure of this optical semiconductor device. The illustrated optical semiconductor device B includes a base plate 101, an LED chip 103 mounted on the base plate, and a reflector 105 surrounding the LED chip 103. As shown in FIG. 8, the reflector 105 is a rectangular frame. As shown in FIG. 9, a space defined by the reflector 105 is filled with a transparent protective resin 106 for covering the LED chip 103 and a wire 104 connected to the chip (the protective resin 106 is not shown in FIGS. 7 and 8). The reflector 105 includes two pairs of inner surfaces (reflecting surfaces) 150a, 150b for light reflection. The two reflecting surfaces 150a are spaced from each other in the direction x, and the reflecting surfaces 150a are parallel to the direction y perpendicular to the direction x. The other two reflecting surfaces 150b are spaced from each other in the direction y, and the reflecting surfaces 150b are parallel to the direction x. These four reflecting surfaces 150a, 150b are inclined relative to the direction z (which is perpendicular to both of the directions x and y). The inclined angle is 45°, for example. The LED chip 103 is a rectangular parallelepiped and includes four side surfaces 103a. Each of the side surfaces 103a faces a corresponding one of the four reflecting surfaces 150a, 150b, and is parallel to either the direction x or y.
Patent Document 1: JP-A-2000-183407
When a predetermined drive voltage is applied to the LED chip 103, light is emitted from the upper surface and the four side surfaces 103a of the chip. The light emitted from the upper surface directly irradiates an illuminated area placed in front of the chip 103 (above the chip 103 in FIG. 9). As shown in FIG. 9, the light emitted from the side surfaces 103a is upwardly reflected by the reflecting surfaces 150a or 150b, and then irradiates the illuminated area. Due to this structure, the illuminated area can be irradiated more efficiently than in a structure where light to be directed toward the illuminated area is emitted only from the upper surface of the chip 103.
FIG. 10 illustrates the optical semiconductor device B used as a light source for backlighting a liquid crystal display. Specifically, the liquid crystal display includes a liquid crystal panel 160 and a light guiding plate 161 provided behind the panel (below the panel in FIG. 10) The optical semiconductor device B faces and is attached to one side surface of the light guiding plate 161. With this arrangement, the width Dw (see FIG. 8) of the optical semiconductor device B needs to be almost the same as (or smaller than) the thickness of the light guiding plate 161, so that the light emitted from the optical semiconductor device B is properly guided into the light guiding plate 161. Thus, when the thickness of the light guiding plate 161 is small, the width of the optical semiconductor device B and consequently the width of the reflector 105 need to be correspondingly small as shown in FIG. 11. However, in such a case, following problem may be caused.
Specifically, when the width of the reflector 105 is small, the inclined angle of the reflecting surfaces 150b can not be made large. Therefore, as shown in FIG. 12, the light emitted laterally from the LED chip 103 does not travel vertically upward by only one reflection at one of the reflecting surfaces 150b, but repeats reflection at two reflecting surfaces 150b and shifts upward gradually. As it is technically difficult to obtain a 100% optical reflectivity at the reflecting surfaces 150b, the light may be attenuated due to the repeated reflection. As a result, the luminance of light emission at the optical semiconductor device may be reduced.